1. Field of the Invention
The invention relates to an oscillator/mixer circuit.
2. Description of the Related Prior Art
Such an oscillator/mixer circuit for processing analog signals is known in the form of a series circuit for an oscillator circuit with a mixer circuit.
In analog circuit technology, a voltage-controlled oscillator is used for producing a signal of a frequency prescribed by setting the dimensions of the oscillator circuit. Using a mixer circuit, particularly a switching mixer, such as a Gilbert cell, a signal to be mixed is mixed with a local oscillator signal, as explained in more detail below, i.e. frequency conversion is performed for the signal which is to be mixed.
Both a voltage-controlled oscillator and a switching mixer in a microelectronic circuit are currently used in a large number of appliances and systems, for example for wireless radio transmission of information, such as in a mobile radio system, a GPS receiver and a television satellite receiver.
FIG. 2 shows the basic design of a normal oscillator 200, as used in microelectronics.
The oscillator 200 has a differential multivibrator 201 having two MOS field-effect transistors 202, 203. In addition, the oscillator 200 shown in FIG. 2 has a resonator element 204, 205 and also at least one voltage-controlled tuning element 206, 207. The voltage-controlled tuning element 206, 207 is also simply called a tuning element.
FIG. 3 shows a basic circuit for an oscillator based on the prior art.
The oscillator 300 is in the form of an RC oscillator and has two MOS field-effect transistors 304, 305 coupled to an input 301 via their respective gate connection 302, 303. The source connection and the drain connection of the first MOS field-effect transistor 304 are connected together and are coupled to a first output 306. The drain connection and the source connection of the second MOS field-effect transistor 305 are likewise coupled to one another and to a second output 307 of the oscillator 300.
The first output 306 is coupled to the gate connection 309 of a third transistor 310 via a first capacitor C 308.
The second output 307 is coupled to the gate connection 312 of a fourth transistor 313 via a second capacitor C 311.
The gate connection 309 of the third transistor 310 is also coupled to the supply connection 315, to which the operating voltage VDD for the oscillator circuit 300 can be applied, via a first electrical resistor R1314. In addition, the gate connection 309 of the third transistor 310 is coupled to the ground potential via a second electrical resistor R2316, which allows the operating point of the oscillator circuit 300 to be set.
The gate connection 312 of the fourth transistor 313 is connected to the supply connection 315 via a third electrical resistor R1317 and also likewise to the ground potential via a fourth electrical resistor 318.
In addition, the drain connection of the third transistor 310 is coupled to the supply connection 315 via a fifth electrical resistor 319, and the drain connection of the fourth transistor 313 is coupled to the supply connection 315 via a sixth electrical resistor 320. The fifth electrical resistor 319 and the sixth electrical resistor 320 are load resistors.
In addition, the source connections of the third transistor 310 and of the fourth transistor 313 are coupled to the ground potential.
FIG. 4 shows the design of a normal Gilbert cell 400, which is an example of a switching mixer, i.e. of a mixer circuit which, as described below, has two transistors, in general terms two switch elements, which are each operated by virtue of the transistors being turned on and off. In addition, the normal Gilbert cell 400 has an analog input transistor.
The Gilbert cell 400 has a first switching transistor 401 and a second switching transistor 402.
The gate connection 403 of the first switching transistor is coupled to a first local oscillator input 404. The first local oscillator input 404 has a first local oscillator signal LO+ applied to it which is thus applied to the gate connection 403 of the first switching transistor 401, in order to control it by means of the local oscillator signal LO+, i.e. to turn it on and off.
The drain connection 405 of the first switching transistor 401 is coupled to a first output 406. In addition, the drain connection 405 of the first switching transistor 401 is coupled to a supply connection 408 via a first electrical resistor 407 as a mixer load resistor. The supply connection 408 can have the operating voltage VDD for operating the mixer circuit 400 applied to it.
Connected to the gate connection 409 of the second switching transistor 402 is a second local oscillator connection 410 to which a second local oscillator signal LO− can be applied for controlling the second switching transistor 402. The second local oscillator signal LO− is shifted through 180° with respect to the first local oscillator signal LO+.
The drain connection 411 of the second switching transistor 409 is coupled to a second output 412 and also to the supply connection 408 via a second electrical resistor 413 as a mixer load resistor.
The source connections 414, 415 of the first switching transistor 401 and of the second switching transistor 402 are coupled to the drain connection 417 of an analog input transistor 418. The gate connection 419 of the input transistor 418 is coupled to a mixing signal input 420 to which the analog input signal ZE to be mixed can be applied. The source connection 421 of the input transistor 418 is coupled to the ground potential.
In order to form an oscillator/mixer circuit from the known circuits described above, i.e. the oscillator circuit 300 and the mixer circuit 400, it is known practice to connect these circuits in series, i.e. a known oscillator/mixer circuit 500 (cf. FIG. 5) has the oscillator circuit 300 described in FIG. 3 and the mixer circuit 400 described in FIG. 4, which are connected to one another in series such that the first output 306 of the oscillator 300 is coupled to the first local oscillator input 404 of the mixer circuit 400 via a first electrical connection 501. The second output 307 of the oscillator circuit 300 is coupled to the second local oscillator input 410 of the mixer circuit 400 via a second electrical coupling 502. The local oscillator signals are shifted through 180° with respect to one another.
The known oscillator/mixer circuit 500 has several drawbacks.                Connecting two complete, essentially mutually independent circuits, i.e. the oscillator circuit 300 and the mixer circuit 400, in series requires a relatively large number of electrical components, i.e. a relatively large number of electrical resistors and transistors, normally MOS field-effect transistors. Integration of the known oscillator/mixer circuit 500 on a chip thus has a considerable requirement in terms of chip area in order to implement it.        Another considerable drawback of the oscillator/mixer circuit 500 can be seen in that two electrical connections 501, 502 are required between the circuit components, i.e. the oscillator circuit 300 and the mixer circuit 400. Particularly in the area of very high-frequency applications, the electrical connections 501, 502 represent a considerable restriction in terms of their usability on account of the very high attenuation to which the output signal produced at the outputs 306, 307 of the oscillator circuit is subject before it reaches the inputs 404, 410 of the mixer circuit 400. In the case of a very high-frequency application in which a signal frequency in a range of approximately 1 GHz to several tens of GHz, preferably up to 77 GHz and above, is required, this results in such an oscillator/mixer circuit 500 barely being able to be used, since no signal with sufficient signal amplitude is applied to, i.e. arrives at, the local oscillator inputs 404, 410 of the mixer circuit 400.        In addition, particularly within the scope of implementing the oscillator/mixer circuit 500 as an integrated circuit, the “matching” problem is of great significance on account of the physical distances between the individual components of the circuit, since it is a considerable problem, technologically, to produce the components which are to be used with sufficient similarity over a relatively long distance from one another on a chip or on a wafer. This means that the components are frequently matched to one another only with great difficulty and sometimes not at all, resulting in an oscillator/mixer circuit 500 which works only poorly or not at all.        
It is also known practice to use a diode or an electrical circuit containing a diode as a mixer circuit or as an oscillator circuit, where, in such a case in which a mixer circuit's mixing is based on an active component, mixing of the signals which are to be mixed is based on the nonlinear characteristic curve of an active component, in the specific case of the diode.
The invention is thus based on the problem of specifying an oscillator/mixer circuit which has a reduced area requirement, when producing it on a chip, as compared with the known oscillator/mixer circuit and where the oscillator/mixer circuit's mixer circuit is not based on the use of the nonlinear characteristic curve of an active component.
The problem is solved by the oscillator/mixer circuit having the features of the independent patent claim.
The oscillator/mixer circuit has an oscillator circuit and a mixer circuit. The oscillator circuit and the mixer circuit have the same components to some extent.
Expressed in other words, this means that the same components in the oscillator/mixer circuit are used both for frequency generation and for the mixing operation.
In addition, the mixer circuit has at least a first switch element and a second switch element, where an output of the first switch element is coupled to a control input of the second switch element via a first feedback element. An output of the second switch element is coupled to a control input of the first switch element via a second feedback element. The feedback elements are set up such that oscillator operation of the oscillator/mixer circuit is made possible.
Oscillator operation of the oscillator/mixer circuit is ensured, by way of example, by virtue of oscillation buildup being ensured in the oscillator circuit in the oscillator/mixer circuit, which can be achieved, by way of example, by virtue of the feedback elements ensuring that the control signals applied to the control inputs of the switch elements are certain to have a phase angle which is suitable for the oscillator circuit's respective circuit type, and also a loop gain of greater than 1.
Expressed clearly, this means that the oscillator/mixer circuit has an oscillator circuit and a mixer circuit which have common components and where the mixer elements are in the form of a switching mixer, i.e. in the form of a circuit which normally has at least two transistors operated in turn-on mode and turn-off mode.
This ensures that the mixing of signals in the oscillator/mixer circuit is not based on a nonlinear characteristic curve of an active component.
The oscillator circuit can also be set up as an LC oscillator circuit.
In this case, in one refinement of the invention, a respective inductor is coupled to the output of the first switch element and to the output of the second switch element, and the input of the first switch element is coupled to the input of the second switch element and to a mixing signal input to which a signal to be mixed can be applied.
Clearly, the oscillator/mixer circuit can thus be regarded as an oscillator circuit in which the ground connection and the source connections of the oscillator transistors in the oscillator circuit have an input transistor connected between them whose gate connection is coupled to the mixing signal input. In general terms, the drain connections of the oscillator transistors and the ground connection have an input transistor connected between them whose control input is coupled to the mixing signal input.
Thus, an oscillator/mixer circuit has been provided which has a significantly reduced number of required components and can simultaneously be operated as an oscillator circuit and as a mixer circuit.
The compact design and the, to some extent, common use of some components both by the oscillator circuit and by the mixer circuit in the oscillator/mixer circuit result in a significant reduction in required chip area for integrating the oscillator/mixer circuit.
In addition, the compact design of the oscillator/mixer circuit means that it is very well suited to a radio-frequency application, in particular, since the signals to be processed need to cover significantly reduced distances over electrical lines as compared with the known oscillator/mixer circuit described above, and are thus subject to significantly reduced attenuation.
In addition, the matching problem for the components used is avoided.
In one refinement of the invention, an analog input transistor is connected between the inputs of the switch elements and the mixing signal input, with the mixing signal input being coupled to the control input of the input transistor.